The present invention relates generally to an apparatus and method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to an apparatus and method for generating a substantially axisymmetric static magnetic field having long, straight contour lines in the resonance region.
It is well recognized that particles of an earth formation having non-zero nuclear spin magnetic moment, for example protons, have a tendency to align with a static magnetic field imposed on the formation. Such a magnetic field may be naturally generated, as is the case for the earth""s magnetic field, BE. After an RF pulse applies a second oscillating magnetic field B1, transverse to BE, the protons will tend to precess about the BE vector with a characteristic resonance or Larmor frequency xcfx89L which depends on the strength of the static magnetic field and the gyromagnetic ratio of the particle. Hydrogen nuclei (protons) precessing about a magnetic field BE of 0.5 gauss, for example, have a characteristic frequency of approximately 2 kHz. If a population of hydrogen nuclei were made to precess in phase, the combined magnetic fields of the protons can generate a detectable oscillating voltage, known to those skilled in the art as a free induction decay or a spin echo, in a receiver coil. Hydrogen nuclei of water and hydrocarbons occurring in rock pores produce nuclear magnetic resonance (NMR) signals distinct from signals arising from other solids.
U.S. Pat. No. 4,717,878 issued to Taicher et al. and U.S. Pat. No. 5,055,787 issued to Kleinberg et al., describe NMR tools which employ permanent magnets to polarize hydrogen nuclei and generate a static magnetic field, B0, and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, and permeability of a formation. The atomic nuclei align with the applied field, B0, with a time constant of T1. After a period of polarization, the angle between the nuclear magnetization and the applied field can be changed by applying an RF field, B1, perpendicular to the static field B0, at the Larmor frequency fL=xcex3B0/2xcfx80, where xcex3 is the gyromagnetic ratio of the proton and B0 designates the static magnetic field strength. After termination of the RF pulse, the protons begin to precess in the plane perpendicular to B0. A sequence of refocusing RF pulses generates a sequence of spin-echoes which produce a detectable NMR signal in the antenna.
U.S. Pat. No. 5,557,201 describes a pulsed nuclear magnetism tool for formation evaluation while drilling. The tool includes a drill bit, drill string, and a pulsed nuclear magnetic resonance device housed within a drill collar made of nonmagnetic alloy. The tool includes a channel, within the drill string and pulsed NMR device, through which drilling mud is pumped into the borehole. The pulsed NMR device comprises two tubular magnets, which are mounted with like poles facing each other, surrounding the channel, and an antenna coil mounted in an exterior surface of the drill string between the magnets. This tool is designed to resonate nuclei at a measurement region known to those skilled in the art as the saddle point.
Great Britain Pat. App. No. 2 310 500, published on Aug. 27, 1997, describes a measurement-while-drilling tool which includes a sensing apparatus for making nuclear magnetic resonance measurements of the earth formation. The NMR sensing apparatus is mounted in an annular recess formed into the exterior surface of the drill collar. In one embodiment, a flux closure is inserted into the recess. A magnet is disposed on the outer radial surface of the flux closure. The magnet is constructed from a plurality of radial segments which are magnetized radially outward from the longitudinal axis of the tool. The flux closure is required to provide suitable directional orientation of the magnetic field.
The tools developed in the prior art have disadvantages which limit their utility in nuclear magnetic resonance logging applications. Magnet designs of prior art tools do not simultaneously produce a highly axisymmetric static magnetic field with long straight contour lines in the resonance region of the formation under evaluation. These factors adversely affect the NMR measurement given the vertical motion of a wireline tool and the vertical and lateral motion of a logging-while-drilling tool.
The above disadvantages of the prior art are overcome by means of the subject invention for an apparatus and method for generating a substantially axisymmetric static magnetic field having long, straight contour lines in the resonance region. A wireline or logging-while-drilling apparatus within a borehole traversing an earth formation determines a formation characteristic by obtaining a nuclear magnetic resonance measurement. The apparatus produces a static magnetic field, B0, into the formation such that the contour lines generated by the static magnetic field are substantially straight in the axial direction at the depth of investigation where the nuclear magnetic resonance measurement is obtained. An oscillating field, B1, is produced in the same region of the formation as the static magnetic field to obtain the NMR measurement. The apparatus includes at least one magnetically permeable member for focusing the static magnetic field. The magnetically permeable member minimizes variations of the static magnetic field in the formation due to vertical motion of the apparatus while obtaining the nuclear magnetic resonance measurement. Further, the magnetically permeable member may minimize variations of the static magnetic field in the formation due to lateral motion of the apparatus while obtaining the nuclear magnetic resonance measurement. In addition, the magnetically permeable member can add significant, prepolarization by causing the B0 field to have substantial magnitude well ahead of the actual region of investigation which can permit increased logging speed.
The static magnetic field is produced using either an axial, radial, or bobbin magnet design. For the axial design, the static magnetic field is produced by an upper magnet surrounding the carrying means and a lower magnet surrounding the carrying means and axially separated from the upper magnet by a distance such that the contour lines generated by the static magnetic field are substantially straight in the axial direction at the depth of investigation where the nuclear magnetic resonance measurement is obtained. The magnets are axially magnetized giving a radially polarized B0 field in the region of investigation. At least one magnetically permeable member for shaping the static magnetic field is located between the lower magnet and the upper magnet. The static magnetic field has either a low gradient or a high gradient, depending on the separation of the magnets, at the depth of investigation where the nuclear magnetic resonance measurement is obtained.
For the radial design, the static magnetic field is produced by an annular cylindrical array of magnets surrounding the carrying means. The array of magnets comprises a plurality of segments, each segment is magnetized in a direction radially outward from and perpendicular to the longitudinal axis of the apparatus. The magnetically permeable member comprises a section of the carrying means, a chassis surrounding a section of the carrying means, or a combination of the chassis and the carrying means section.
For the bobbin design, the static magnetic field is produced by a plurality of geometrically and axisymmetric magnet rings surrounding the carrying means. The plurality of rings comprises an upper ring, a plurality of inner rings, and a lower ring. The radius of the upper and lower rings is greater then the radius of each inner ring. Each of the plurality of rings is axisymmetrically polarized and the direction of polarization for each ring differs progressively along the ring of magnets. The polarization direction of the upper ring is radially opposite to the polarization direction of the lower ring. The polarization of each inner ring changes progressively such that an angle between the polarization and a transverse radius vector varies linearly for each inner ring.